Apparatus and method for eliminating varying pressure fluctuations in a pressure transducer

ABSTRACT

A differential pressure transducer employing a coiled tube to eliminate varying pressure fluctuations is provided. In one embodiment, a method comprises receiving, at an inlet tube of a dampening chamber, a main pressure, wherein the main pressure includes a static pressure component and a dynamic pressure component; filtering, by the inlet tube, at least a portion of the dynamic pressure component of the main pressure; outputting, from the inlet tube, a first filtered main pressure; receiving, at a volume cavity of the dampening chamber, the first filtered main pressure, wherein the volume cavity is operatively coupled to the inlet tube; filtering, by the volume cavity, at least a portion of the dynamic pressure component of the first filtered main pressure; outputting, from the volume cavity, a second filtered main pressure; and wherein the dampening chamber is tuned to a predetermined resonance frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application claiming priority to U.S.patent application Ser. No. 13/084,850, filed on Apr. 12, 2011 which isa continuation-in-part application of U.S. patent application Ser. No.12/574,587, filed on Oct. 6, 2009, which is a continuation applicationclaiming priority to U.S. patent application Ser. No. 12/151,816, filedon 9 May 2008, now U.S. Pat. No. 7,597,004, issued on Oct. 6, 2010, allof which are entitled “APPARATUS AND METHOD FOR ELIMINATING VARYINGPRESSURE FLUCTUATIONS IN A PRESSURE TRANSDUCER”, and are herebyincorporated in their entirety as if fully set forth herein.

FIELD OF THE INVENTION

This invention relates to pressure transducers and more particularly toa differential pressure transducer employing a coiled tube to eliminatevarying pressure fluctuations.

BACKGROUND OF THE INVENTION

Differential pressure measuring devices usually include one of twodesign variations. In a first design two half bridge circuits areconnected together to form a Wheatstone bridge. The half bridges arenormally provided on a silicon sensing die. In this case, there are twoseparate dies, with a half bridge on each. Each of the devices areported to the main or reference pressure. One device is ported to thereference pressure. For a differential pressure measurement, the halfbridges are electrically connected to electrically subtract the high ormain pressure from the low or reference pressure resulting in a voltageproportional to the difference pressure. These techniques are wellknown. See for example U.S. Pat. No. 6,612,179 issued on Sep. 2, 2003 toA. D. Kurtz and assigned to the assignee herein, namely KuliteSemiconductor Products and entitled Method and Apparatus for theDetermination of Absolute Pressure and Differential Pressure Therefrom.This patent describes the combination of absolute and differentialpressure sensing devices including a plurality of absolute pressuretransducers, each transducer including a plurality of half bridgepiezoresistive structures and a device which selectively couples atleast one of the half bridges to another half bridge.

In other prior art configurations, a single pressure sensing capsule isemployed with the reference pressure ported to the rear side of thesilicon sensing die. The main pressure is ported to the top side of thesilicon sensing die. This design requires the use of a Wheatstone bridgeon a single die. The difference of the main and reference pressureresults in the differential pressure. Again, the differential pressureresults in a voltage output. This design requires the reference tube tobe connected to the reference pressure inlet. In any event, in actualoperation both types of differential pressure measuring devices can besubjected to pump ripple, or a sinusoidally varying pressurefluctuation. Normally pump pressure is desirable to be measured insystems having pumps. For example, in an automobile, the oil thatlubricates the engine of a car must be forced at high pressure aroundchannels in the engine. In order to operate, a pump is used, which pumpnormally is referred to as a gear pump.

The rotating cam shaft of the engine normally powers the oil pump,driving a shaft that turns a pair of intermeshing gear wheels inside aclose fitting chamber. The oil enters the pump where it is trapped bythe wheels. The wheels carry the oil around to the outlet, where theteeth come together as they intermesh. This action squeezes the oil andraises its pressure as it is close to the outlet. The speed of pumpingis directly linked to the speed of the engine. In any event, in such apump, the pressure at the output as well as pressure at the input isnormally monitored. The pressures are monitored by a pressuretransducer. However, these pressure transducers can be subjected to pumpripple or a sinusoidally varying pressure fluctuation. In an adversesituation, the pipe and cavity of the reference or main side of thesensor can be tuned to the frequency of the pump ripple. By thisoccurring, one creates a resonance in the tube which results in anamplified pressure being applied to the transducer. This amplifiedpressure can seriously harm the transducer as will be further explained.

It is also known that the pump ripple is a function of the number ofgear teeth in the pump and the number of revolutions per minute of theteeth. This, as indicated, can vary as the RPM of the pump can vary, andhence such a tube must be selected to filter the range of frequencies toprevent resonance and amplification in the pump operating RPM range. Itis understood that the resonance and amplification of the pump ripplepressure can exceed the rating of the sensing die or pressure capabilityof the structure. Exceeding the rated pressure imparts excessive stresson the die which experiences brittle failure. Aside from loss of thesignal from the sensor, on a filter application, contaminates from thedirty side of the filter can be passed to the clean side, thus furtherdestroying the sensor or equipment downstream. One therefore requires apressure transducer which will operate to eliminate pump ripple or toeliminate varying pressure fluctuations in a sensor and still enable thesensor to be small and compact.

SUMMARY OF THE INVENTION

A differential pressure transducer having a sensor and for providing anoutput proportional to the difference between a main pressure P₁ and areference pressure P₂, wherein one of the pressure inputs contains anundesirable varying pressure fluctuation which fluctuation canundesirably produce excessive stress on said sensor, comprising: saidsensor having a deflectable diaphragm, said diaphragm having pressuresensing elements on said diaphragm which elements provide an outputproportional to an applied pressure on said diaphragm; a first portcommunicating with one surface of said diaphragm to provide a firstpressure thereto; a second port communicating with the other surface ofsaid diaphragm to provide a second pressure thereto, a coiled tube inseries with one of said ports and dimensioned to suppress said varyingpressure fluctuation prior to the application of said associatedpressure to said diaphragm. Alternative embodiments may comprise anadjustable dampening chamber that can comprise a spiral inlet tubeand/or a volume cavity to attenuate unwanted pressure fluctuations inplace of the coiled tube.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a cross-sectional view of a differential transducerand housing having two headers according to the prior art.

FIG. 2 is a cross-sectional view of prior art transducer having oneheader and operative in a differential mode.

FIG. 3 is a diagrammatic view of a pressure transducer having a pipe andcavity and useful for explaining ripple operation.

FIG. 4 is a cross-sectional view of a pressure sensor employing a coiledtube according to this invention.

FIG. 5 is a schematic diagram of a coiled tube according to thisinvention.

FIG. 6 is a cross-sectional view of a silicon sensor die used in thisinvention.

FIG. 7 is a schematic diagram of a Wheatstone bridge such as the typeemployed with the sensor of FIG. 6.

FIG. 8 illustrates an alternative embodiment for attenuating largedynamic pressure waves using an adjustable dampening chamber comprisinga spiral inlet tube and a volume cavity.

FIG. 9 illustrates a cover that can be welded to the adjustabledampening chamber, according to alternative embodiments of the presentinvention.

FIG. 10 illustrates the cover welded to the adjustable dampeningchamber, according to alternative embodiments of the present invention.

FIG. 11 illustrates the adjustable dampening chamber attached to asensor module, according to alternative embodiments of the presentinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a typical prior art differentialpressure transducer. The pressure transducer includes a housing 18. Inthe housing there is a main pressure header 10 and a reference pressureheader 14. Both pressure headers contain piezoresistors sensors whichare accommodated on silicon diaphragms. As one can ascertain, there is amain pressure port 16 to which a main pressure is applied and areference pressure port 15 which a reference pressure is applied. Theoutput of the device is proportional to the difference between the mainpressure and the reference pressure. The design of FIG. 1 uses twopressure sensing capsules 10 and 14. Each capsule contains a half of aWheatstone bridge on an associated silicon sensing die. For adifferential pressure measurement, the half bridges are electricallyconnected to electrically subtract the high pressure from the lowpressure. Each header is ported to the respective pressure port. Header10 is ported to the main pressure port 16 and header 14 is ported to thereference pressure port 15. The half bridges on each of these sensorsare electrically connected to electrically subtract the high pressurefrom the low pressure resulting in a voltage proportional to thedifference in pressure.

Referring now to FIG. 2, there is shown a prior art pressure transducerwhich employs a reference tube and is a differential pressure transduceras indicated above. As seen, the pressure sensor or transducer iscontained in a metal housing 20. There is a header 23 in the housingwhich header contains a single die or a semiconductor sensor 22 whichbasically is a full Wheatstone bridge. As seen, there is a lock nut 24which couples the reference tube 21 to one side of the sensor 22. Thistube 21 receives the reference pressure. The other side of the sensor isexposed to a main pressure port 28. The reference tube couples thereference pressure at port 27 to the other side of the sensor die tocause the Wheatstone bridge to produce an output which is the differencebetween the reference port pressure and the main port pressure.Therefore, as shown in FIG. 2, a single pressure sensor capsule is used,namely capsule 23. In this prior art design, a full Wheatstone bridge ona single die is used. The difference of the main and reference pressureresults in only the differential pressure inducing stress in the sensingdiaphragm.

In any event, as can be seen, FIG. 2 shows a single pressure sensingcapsule with the reference pressure ported to the rear side of thesilicon sensing die and the main pressure ported to the top side. Inthis design a full Wheatstone bridge on a single die is used asindicated. The difference of the main and reference pressure results inthe differential pressure which induces stress in the sensing diaphragm.Again, the differential pressure results in a voltage output from theWheatstone bridge. The design shown in FIG. 2 requires a reference tubeto be connected to the reference pressure inlet. The low delta-pmeasurements of this design offers a much greater accuracy and hence isa preferred design for low pressure inputs. As indicated above, bothdesigns can be subjected to pump ripple or a sinusoidally varyingpressure fluctuation. In an adverse situation, the pipe and cavity ofthe reference pressure on one side of the sensor can be tuned to thefrequency of this pump ripple if this occurs. If this occurs thepressure increases to a level high enough to break or rupture thediaphragm.

Referring to FIG. 3 there is shown a pipe and cavity model which will beemployed to explain how an equation for the resonance frequency of apipe and cavity was derived. As seen in FIG. 3, there is a pipe 30 whichhas a diameter (d) and a length (l). The pipe has a pressure input port31 which interfaces with a cavity 32 having a volume (v). The cavity, aswell as the pressure inlet interfaces with a pressure transducer 33.Using standard system dynamic analysis, an equation was derived for theresonant frequency of a pipe and cavity as shown in FIG. 3. The port ofthe pressure sensor 31 is modeled as a series of pipes representing theorifice and fluid channels, and cavities in front of the sensingcapsules. The Helmholtz equation for the resonant frequency (F_(n)) ofthe pipe/cavity system is:F _(n)=√{square root over ((3πr ² c ²/4lv/2π,)}

where r=internal radius of pipe;

c=velocity of sound in the pressure fluid;

l=length of pipe; and

v=volume of the cavity.

Thus, as indicated above, when the pipe and cavity structure of thepassage is tuned to the pump ripple frequency, the pump ripple pressureis amplified. This resonance and amplification of the pump ripplepressure can exceed the rating of the sensing die or pressure capabilityof the structure. Exceeding the rated pressure applies excessivestresses on the die, which experiences brittle failure. Aside, from lossof the signal from the sensor, on a filter application, contaminatesfrom the dirty side of the filter can be passed to the clean side, thusdestroying the entire sensor or equipment downstream. For large tube orpipe diameters, the resonance is proportional to the radius. As the tubediameter gets smaller, capillary action takes over. As the tube diameterdecreases below 0.040 inch, the change in resonant frequency diminishes.Thus there is a diminishing return with decreasing tube diameter. Inaddition, manufacturability decreases and the likelihood of trappingparticles in the small diameter tube increases. The trapping ofparticles will clog the sensor and will decrease reliability. As can beseen from the above formula, the resonant or critical frequency (Fn) isalso inversely proportional to the square root of the length of thepipe. In many design applications, the frequency can be suppressedmerely by increasing the pipe length. In any event, by increasing thepipe length, one therefore increases the size of the sensor as the pipehas to be accommodated.

Referring to FIG. 4 there is shown a differential sensor using a singlesilicon die (FIG. 2), which silicon die 41 contains a Wheatstone bridgeand where the main pressure from a main pressure port 50 is applied tothe top surface of the die, while a reference pressure 51 is applied tothe bottom surface of the die. As seen, there is a coil 52 whichbasically is in series with the reference port inlet 51. The coil 52 isa tubular coil which essentially consists of a tube which is wound abouta mandril having a screw type thread. In this manner the coil 52 issubstantially decreased in length and now can be positioned inside thesensor. The dimensions of the coil are selected according to the aboveequation and the length of the coil is much longer than the length ofthe reference port and tube. Normally the reference port inlet wouldhave to be expended by the length of the reference tube or the referencetube extended by the expanded length of the coil 52. This would ofcourse create a problem in manufacturing a small sensor. Thus coiling ofthe tube 52 keeps the size minimized to aid compact packaging.

Also shown in the Figure is header 42 which essentially encompasses thesilicon die. There is shown a terminal port 54 which receives leads fromthe silicon sensing die or from the Wheatstone bridge on the siliconsensing die and directs the outputs through cable 53. As seen, apressure would be applied to the main port 50 while the referencepressure would be applied to the inlet port 51. The port 51 would becoupled to a pressure associated with a pressure derived from a pump 60.As indicated above, the pump 60 can be a gear pump or any other pump andwould contain pump ripple. The pump ripple, due to the fact that it canoccur over a fairly wide range of frequency such as three kilohertz (3kHz) cycles to five kilohertz (5 kHz) cycles will cause resonance in thereference pressure path including tube 44. This resonance will causeamplification of the pressure which could result in exceeding the ratingof the sensing die 41. This resonance and amplification of the pumpripple pressure can cause the sensing die to experience brittle failureand therefore destruction.

The coil 52 dimensions are selected based on the equation shown aboveand is maybe wound as indicated on a mandril or on a threaded screw.Typically the coil will have a diameter in the center of approximatelythree-eighth (⅜) inches with a tube having an outer diameter of 0.04inches and an inner diameter of 0.02 inches and a length of two or moreinches. These dimensions indicate a coil capable of suppressing pumpripple frequency between three kilohertz (3 kHz) to four kilohertz (4kHz). It is of course understood that coiled structures have been usedin conjunction with pressure transducers for other applications. Forexample reference is made to U.S. Pat. No. 7,188,528 issued on Mar. 13,2007 and entitled Low Pass Filter Semiconductor Structures for use inTransducers for Measuring Low Dynamic Pressures in the Presence of HighStatic Pressures by A. D. Kurtz, et al, an inventor herein, and assignedto Kulite Semiconductor Products, Inc. That patent shows a long tubewhich basically acts as a low pass filter and will only pass frequencieswhich are below one hundred twenty Hertz (120 Hz). In this manner, thedynamic frequency which is five kilohertz (5 kHz) or greater will notpass through the tube. That patent, as indicated, shows a tube foroperating as a low pass filter. It is also noted that the tube is not inany manner inserted into the pressure transducer as the tube will be toolong to be conveniently employed. Reference is also made to U.S. Pat.No. 7,107,853 issued on Sep. 19, 2006 to A. D. Kurtz, an inventorherein, and entitled Pressure Transducer for Measuring Low DynamicPressures in the Presence of High Static Pressures. This patent is theparent application of the above noted patent, both of which areincorporated herein in their entirety. Thus there has been described acoil transducer which will operate to suppress pump ripple and preventthe pump ripple from being amplified and thus destroying the sensing dieof a semiconductor pressure transducer.

FIG. 5 shows a coiled tube 52 having a length (L) of 0.4 inches, a tubediameter (d) of 0.04 inches and a coil diameter (D) of three-eighth (⅜)inches.

Referring to FIG. 8, an alternative embodiment for attenuating largedynamic pressure waves caused by pump ripple is illustrated. Thisalternative embodiment can be used in place of the coiled tube 52described above. This embodiment comprises an adjustable dampeningchamber 800 that may include an inlet tube 805 having a first end and asecond end and a volume cavity 810. The first end of the inlet tube 805can be adapted to receive a fluid having a main or reference pressureand the volume cavity 810 can be adjacent the second end of the inlettube 805. A sensor module can be attached to the end of the volumecavity opposite the second end of the inlet tube. The inlet tube 805 canbe configured into a spiral shape and further, can be machined from aplate or shaped out of a straight elongated tube. As large dynamicpressure waves propagate through the fluid having a main or referencepressure, the inlet tube 805 and the volume cavity 801 dampen the largedynamic pressure waves (caused by pump ripple) before they reach andcause damage to the sensor module. Other exemplary embodiments, forexample, can comprise a plurality of inlet tubes and/or volume cavitiesin series to achieve desired attenuation characteristics.

This alternative embodiment allows for adequate attenuation of dynamicpressure waves and enables accurate measurement of main, reference,and/or differential pressure within the system. The resonance frequencyof the adjustable dampening chamber 800 can be tuned using the Helmholtzequation, defined above. Based on the Helmholtz equation, discussed indetail above, the length of the inlet tube 805, the diameter of theinlet tube 805, and the volume of the volume cavity 810 are parametersthat can be manipulated to achieve a desired resonance frequency. Oneskilled in the art will appreciate that it is desirable to tune theadjustable dampening chamber 800 to an appropriate resonance frequencythat dampens unwanted, destructive pressure waves but enables pressurewaves to be measured by the sensor module (i.e., main or referencepressure) to pass through. Therefore, an appropriate interplay betweeninlet tube length, inlet tube diameter, and cavity volume must be madeto achieve this balance. For example, the inlet tube 805 and volumecavity 810 can be tuned to attenuate large dynamic pressure waves ofabout one kilohertz (1 kHz) and higher and accurately pass throughslower oscillating pressures of about one hundred Hertz (100 Hz) andlower.

The cross-section of the inlet tube 805 can be rectangular, circular, ormany other geometrical shapes. In exemplary embodiments, wherein thecross-section of the inlet tube 805 is circular, the diameter can beabout five thousandth of an inch (5 mils) to about fifty thousandth ofan inch (50 mils) or larger. The length of the inlet tube 805 can rangefrom about 0.25 inches to about five (5) inches. This configurationprovides a compact inlet tube 805, which is important for maintainingthe miniaturized size of the overall transducer system. One skilled inthe art will appreciate that the geometrical configuration of the inlettube 805 can be determined using the Helmholtz equation. The geometricalconfiguration of the inlet tube 805 can be tuned such that the resonanceis well below the frequency of the dynamic pressure waves caused by pumpripple within the system. For example, if the system is experiencingdynamic pressure waves in a fluid at a frequency of fifteen hundredHertz (1.5 kHz), the inlet tube 805 can be designed using the Helmholtzequation such that its resonance is about three hundred Hertz (300 Hz)to five hundred Hertz (500 Hz).

A cover 815, as illustrated in FIG. 9, can be welded to the adjustabledampening chamber (for example, the adjustable dampening chamber 800 asshown in FIG. 8), which forces the pressure waves to propagate throughthe adjustable dampening chamber, and more specifically propagatethrough the inlet tube (for example, the inlet tube 805 as shown in FIG.8). Referring to FIG. 10, there is shown the cover 815 aligned andwelded to the inlet tube 805. Referring to FIG. 11, there is shown theadjustable dampening chamber 800 attached to a sensor module 110.

Referring to FIG. 6 there is shown a schematic cross-sectional view of atypical sensor module. The sensor module 60 contains a semiconductorsubstrate having a thin active area or diaphragm 63 upon whichpiezoresistors such as 61 and 62 are positioned. Such devices areextremely well known and the prior art is replete with semiconductordies or semiconductor sensors using piezoresistors as 61 and 62 to forma Wheatstone bridge configuration. While two piezoresistors are shown,it is understood that there are normally four (4) piezoresistors. Thesensor can be protected by coating it with a layer of silicon dioxideand essentially the pressure P₁ is applied to the top of the sensoractive area as shown. The pressure may be transmitted to the sensor byan oil-filled cavity which is positioned above the sensor, as is alsowell known. In any event, the sensor has the pressure P₁ applied to thetop side, which for example, may be the main pressure as applied to port50 of FIG. 4. In any event, the reference pressure P₂, which for exampleemanates from the pump 60 of FIG. 4 is applied to the underside of thediaphragm. The Wheatstone bridge or sensor provides an output which isequal to P₁−P₂ which is the differential pressure. As seen in FIG. 6,the device is shown in FIG. 4 as sensor 41. Thus the sensor 41 or sensor60 of FIG. 6 receives a pressure P₁ on the top side and pressure P₂ onthe bottom side. As indicated and shown, pressure P₂ is derived from agear pump 60 which may exhibit pump ripple, which ripple is suppressedby the coil 52 of FIG. 5 as explained in conjunction with FIG. 4.

As shown in FIG. 7 there is a Wheatstone bridge configuration which is atypical sensor structure. The Wheatstone bridge, for example, has fourresistors which can be piezoresistors as resistor 91 and so on. Thepiezoresistors change resistance according to an applied pressure. Asseen there are five leads associated with the bridge. Two are used forbiasing the bridge and three for providing an output. These leads asshown in FIG. 4 are directed out from the device via cable 53. Thus asshown above, there is a low pressure differential transducer whichoperates in conjunction with a coil to suppress pump ripple andtherefore enable reliable operation during the presence of such rippleor other disturbing variations. This results in improved operation ascompared to prior art devices while enabling one to make a extremelysmall transducer structure.

It should be apparent to one skilled in the state of the art that thereare many alternate embodiments which can be determined or are deemed tobe encompassed within the spirit and scope of the claims appendedhereto.

The invention claimed is:
 1. A method, comprising: tuning an adjustabledampening chamber for a predetermined resonance frequency, wherein theadjustable dampening chamber comprises a machined plate inlet tube, amachine plate inlet tube cover having an inlet hole, and an adjustablevolume cavity in communication with the inlet tube, wherein the tuningcomprises: selectively controlling a length of the inlet tube byalignment of the inlet hole of the machine plate inlet tube cover withrespect to the machined plate inlet tube; controlling a cross-sectionaldiameter of the inlet tube; and controlling a volume of the adjustablevolume cavity; installing, in a housing of a differential pressuresensor, the adjustable dampening chamber, wherein the differentialpressure sensor comprises a diaphragm, and wherein the housing defines amain pressure port and a reference pressure port; configuring the mainpressure port to be in communication with a first surface of thediaphragm; configuring the reference pressure port to be incommunication with the inlet tube, wherein the reference pressure portis adapted to receive a reference pressure having a static pressurecomponent and a dynamic pressure component, and wherein the inlet tubeis configured to filter at least a portion of the dynamic pressurecomponent of the reference pressure and to output to the adjustablevolume cavity, a first filtered reference pressure; wherein theadjustable volume cavity is tuned to reduce at least a portion of thedynamic pressure component of the first filtered reference pressure andto output to a second surface of the diaphragm, a second filteredreference pressure.
 2. The method of claim 1, further comprising:coupling a transducer to the diaphragm for measuring a differencebetween the main pressure and the second filtered reference pressure. 3.The method of claim 1, wherein the inlet tube is machined in a spiralshape.
 4. The method of claim 1, wherein tuning the adjustable dampeningchamber further comprises: machining a shape on a surface of theadjustable dampening chamber; and securing a cover over the machinedshape to form at least a portion of the inlet tube.
 5. The method ofclaim 1, wherein the adjustable dampening chamber comprises one piece,and wherein the adjustable volume cavity and the inlet tube areintegrated in the adjustable dampening chamber.
 6. The method of claim1, wherein the adjustable dampening chamber is tuned to thepredetermined resonance frequency using a Helmholtz equation.
 7. Themethod of claim 1, wherein the inlet tube is defined by across-sectional diameter ranging from about five thousandth of an inch(5 mils) to about fifty thousandth of an inch (50 mils).
 8. The methodof claim 1, wherein the inlet tube has a length of about twenty-fivehundredths (0.25) of an inch to about five (5) inches.
 9. The method ofclaim 1, wherein a cross-section of the inlet tube is circular.
 10. Themethod of claim 1, wherein the dynamic pressure component includesfrequencies greater than one hundred Hertz (100 Hz).
 11. The method ofclaim 1, wherein the volume cavity includes an array of volume cavities.